Process and composition for pickling steel



United States Patent O 3,385,734 PROCESS AND COMPOSITION FOR PICKLING STEEL Lloyd B. Barkley, Pittsburgh, and Robert G. Buckingham, Finleyville, Pa., assignors to Pennsylvania Industrial Chemical Corporation, a corporation of Pennsylvania No Drawing. Filed Dec. 2, 1964, Ser. No. 415,493 5 Claims. (Cl. 134--3) In the production of cold reduced strip steel it is necessary that the oxides, commonly called mill scale, that form during hot rolling be removed to prepare the strip for further processing, for instance by drawing operations or to condition the strip to receive various types of surface coatings. For many years this has been accomplished by pickling the hot rolled strip in solutions of sulfuric acid. In that practice the hot rolled material is passed through a tank, or a series of tanks, called a pickling line, countercurrent to the movement of the pickling solution, with strong fresh acid being added to the end of the pickling line where the steel exits.

Sulfuric acid pickling entails certain disadvantages. For instance, the oxides of mill scale are converted to ferrous sulfate which builds up in the pickling liquor, and a high concentration of that salt at the disharge end of the pickling tank is detrimental to the pickling operation. Moreover, the disposal of spent acid presents major problems. The spent liquor may contain as much as 5 percent of free acid and as much as percent of ferrous sulfate. Obviously, such a solution cannot be dumped into streams, lakes or other natural waters. The free acid may be new tralized, as with lime, and the ferrous sulfate may be removed by concentration of the spent liquor to cause deposidon of the salt, to prepare the liquor for harmless discharge into natural waters. Such procedures are expensive, and even if the spent liquor merely be discharged into sumps the necessary land represents a cost burden and there is danger that the acid and sulfate will percolate and render ground waters useless. The ferrous sulfate content of spent liquor represents an economic loss but its recovery as such or its conversion to iron oxide are not justified because of their low market value.

Pickling with hydrochloric acid has been proposed and tried but although it has found limited use in the batch pickling of steel sheet and plate, it has not supplanted sulfuric acid in continuous strip pickling lines. One reason for this is that the fumes given off during hydrochloric acid pickling are highly objectionable to workmen and create an atmosphere that corrodes metallic equipment.

It is among the objects of this invention to provide a method of pickling steel with hydrochloric acid in the practice of which fuming is suppressed to a low level, which is productive of a bright and white surface the irregularities of which are much less pronounced than those produced by sulfuric acid, which results in lower residual surface chloride as compared with other pickling methods, and which is adapted particularly to continuous operation with conventional pickling equipment.

Another object is to provide a novel hydrochloric acid pickling bath for practicing the foregoing method, which embodies an inhibiting agent and an agent for complexing the iron chloride resulting from the action of the pickling solution.

Still another object is to provide a concentrate for making baths in accordance with the invention.

Other objects will be recognized from the following description.

In accordance with this invention there is provided a pickling concentrate comprising a solution of, by weight about 80 to 85 percent of B. hydrochloric acid (average 31.4 percent HCl), and about 10 to 15 percent of ice added water, about 1 to 3 percent of sodium dihydrogen phosphate monohydrate, about 0.5 to 1.5 percent of water soluble polyethylene glycol. and about 0.2 to 1.5 percent of monoethanolamine.

We have found that these composidons when diluted with water to about 10 to 15 percent HCl concentration not ony effect rapid removal of mill scale but also exert a combination of other desirable functions, presumably through synergistic action. Since the steel is not attacked the problem of hydrogen evolution with H 80 pickling is avoided. The solutions attack the scale in such manner that the use of scale breakers is unnecessary. The steel has a bright surface with no dark deposit. Of particular advantage is the fact that the surface is very much smoother than is obtained with sulfuric acid pickling. Measurement of surface irregularities of strip steel pickled in accordance with this invention shows that they average about to microns as compared with to micron irregularities at the surface of sulfuric acid pickled steel. Furthermore, after the strip has been washed there is no detectable chloride at the surface.

In addition to these advantages there is very low fume level in the vicinity of the pickling tank, which is particularly desirable from the standpoint of labor relations and also reduced attack of neighboring metal equipment. This we believe to be due to an inhibiting action of the ethanolamine. The phosphate content of these solutions exerts a complexing action upon the iron chloride formed and insures the formation of a bright finish.

An important feature of the invention is that the concentrate provided by the invention rriay be used to form the bath by dilution with water and thus avoid the necessity of shipping the large quantities of water to the steel plant if the bath were to be made up ready for use and shipped to the point of use. In practice the concentrate of this invention is blended with about 5 to 15 parts of 20 B. HCl and then diluted with water either by the addition of water or by the condensation of steam used to heat the bath, so that the most concentrated tank of the pickling line, from which the steel exits, will contain about 10 to 15 percent of HCl. The other tanks of the line will contain varying amounts of acid, depending on the number of tanks, the rate of travel of the steel strip, bath temperature, and related factors. The actual concentration of acid is maintained by the operators according to plant conditions by addition of the concentrate.

The polyethylene glycol serves, we now believe, to increase the solubility in concentrated hydrochloric acid of the phosphate salt and thus permits the formulation of the concentrate that characterizes this invention and which in turn avoids the shipment of large quantities of water.

In the production of the concentrate it is preferred to use a polyethylene glycol such as one of about 6,000 average molecular weight, which is a waxy solid at room temperature. The monoethanolamine may be replaced by diethanolamine or triethanolamine, and mixtures of any two or all three of them may be used likewise. Instead of the phosphate specified for the bath it may be supplied to the concentrate as sodium chloride and orthophosphoric acid in amounts equivalent to the specified amount of sodium dihydrogen phosphate monohydrate.

Conventional pickling equipment is used in the practice of this invention. There is nothing critical in the temperature of the bath but generally it should range from about F. to not over about 210 F. The pickled strip is treated conventionally by subjecting it to a cold water spray followed by a hot water spray and then flash drying it at about 230 F.

In accordance with conventional pickling practice the steel moves countercurrently to the direction of fiow of the pickling liquor with makeup acid being supplied by concentrate added at the end of the tank from which the strip exits.

According to the provisions of the patent statutes, we have explained the principle of our invention and have described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Weclairn:

1. In a method of pickling steel the steps of blending about 5 to percent of B. HCl with a pickling bath concentrate consisting essentially of, by weight, about to percent of 20 B. hydrochloric acid and about 10 to 15 percent additional water, about 1 to 3 percent of sodium dihydrogen phosphate monohydrate, about 0.5 to 1.5 percent of polyethylene glycol, and about 0.2 to 1.5 percent of at least one ethanolamine, diluting the blend with water to about 8 to 15 percent of HCl, and passing the steel through the resultant bath.

2. Method according to claim 1, said glycol being of about 6,000 molecular weight.

3. Method according to claim 1. said ethanolaminebe-= ing monoethanolamine.

4 4. concentrate for preparing a steel pickling bath consisting essentially of, by weight, about 80 to 85 percent of 20 B. hydrochloric acid, about 1 to 3 percent of sodium dihydrogen phosphate monohydrate, about 0.5 to

1.5 percent of poly -thyiene glycol, about 0.2 to 1.5 percent of an ethanolamine, and about 10 to 15 percent of added water.

5. Concentrate according to claim 4, said ethanolamine being monoethanolamine.

References Cited 20 REUBEN FRIEDMAN, Primary Examiner.

J. L. DE CESARE, Assistant Examiner 

1. IN A METHOD OF PICKLING STEEL THE STEPS OF BLENDING ABOUT 5 TO 15 PERCENT OF 20* BE. HCL WITH A PICKLING BATH CONCENTRATE CONSISTING ESSENTIALLY OF, OF WEIGHT, ABOUT 80 TO 85 PERCENT OF 20*BE. HYDROCHLORIC ACID AND ABOUT 10 TO 15 PERCENT ADDITIONAL WATER, ABOUT 1 TO 3 PERCENT OF SODIUM DIHYDROGEN PHOSPHATE MONOHYDRATE, ABOUT 0.5 TO 1.5 PERCENT OF POLYETHYLENE GLYCOL, AND ABOUT 0.2 TO 1.5 PERCENT OF AT LEAST ONE ETHANOLAMINE, DILUTING THE BLEND WITH WATER TO ABOUT 8 TO 15 PERCENT OF HCL, AND PASSING THE STEEL THROUGH THE RESULTANT BATH. 